Unlock Max 300:
You can unlock Max 300 in more ways the one. One method is to get an AAA ranking on any song. You can also unlock Max 300 by using the roulette until you land on it.

Unlock Songs:
To unlock the songs listed, complete the corresponding criteria:
Colors (For Extreme): Play 100 songs.
Graduistic Cyber: Get an AA rank on any song.
Healing Vision (Angelic Mix): Get at least a C rating on each difficulty of Healing Vision.
Max300: Get a AAA on any song.
MGS 2 Mission R: Get an A rank on any song.
Orion .78 (Civilization Mix): Play at least 300 songs.
Paranoia Rebirth: Get at least a C rating on each difficulty of Paranoia Eternal.
Put Your Faith in Me (Saturday Night Mix): Fail a song on purpose.
Sana Mollete Ne Ente (BLT Style): Get at least a C rating on each difficulty of Sana Mollete Ne Ente.